The invention relates to orientation switchable optical fibres, otherwise known as in-fibre switches, for converting light in one orientation or polarisation to one of two or more possible orientations.
In typical optical communications networks, modulated optical signals are transported by optical fibres. Within the network, usually at nodes or intersections, various signal processing operations are carried out. Certain processes require the polarisation or orientation of light in one or one of two or more particular axes. Examples of such processes are polarisation routing, polarisation control, PMD compensation, pump combination, detector optimisation and bit interleaving.
Conventional step-index optical fibres typically comprise a central core of relatively high refractive index material, surrounded by a cladding of relatively low refractive index material, encased in a protective jacket. Light is confined to the core by total internal reflection as a result of the step difference in refractive index between the core and the cladding.
In contrast, photonic crystal fibres (PCFs) comprise only one material which is provided with periodic features, generally air holes, surrounding a central, so-called defect. The periodic features define a lattice, and the fibre is characterised by the lattice pitch and the diameter of the periodic features.
In the case of solid core PCFs, the central defect has a relatively high refractive index in comparison to the surrounding material, whose effective refractive index is reduced as a result of the presence of the periodic features, so that, in a manner analogous to a step-index fibre, light is confined to the central defect. The defect may be provided by continuous material at the centre of the fibre and the periodic features by air holes in the same material. Thus, a solid core PCF has a central part of fibre material running the whole length of the fibre, and a lattice of regularly sized and spaced air channels also running the whole length of the fibre. Such PCFs are often known as holey fibres.
In optical communications networks, in-fibre switching would be advantageous because, at the least, it avoids the need for non-fibre switching entities in the network. The applicants are not aware of any communications network optical fibre which is provided with the means to switch light it is carrying from one orientation to another. In other words, the applicants do not know of any in-fibre switch currently available.
An object of the invention to provide an optical fibre which is orientation switchable.
According to a first aspect, the invention provides an orientation switchable optical fibre comprising an elongate body having two ends, which body is twisted intermediate the two ends, a central core region in which light carried by the fibre is confined, passive means extending substantially from one end to substantially the other end of the body, which passive means promotes orientation of the light in at least one first transverse axis of the body, active means extending substantially from the location of the twist to substantially the other end of the body, which active means is switchable between an off state in which the active means is ineffective and an on state in which the active means is effective to promote orientation of light in at least one second transverse axis of the body, wherein in the off state of the active means light remains confined in the at least one first axis along the length of the body and in the on state of the active means light transits as on passing the location of the twist from the first axis to the at least one second axis.
In the absence of a twist, the light would not transit from the at least one first axis to the at least one second axis as the light would never xe2x80x9cseexe2x80x9d the other axis. The twist is preferably through 90xc2x0 although other angles of twist are equally applicable.
The body may be substantially circular in cross section and each at least one transverse axis may be diametrical axes. In the case of only one first axis and only one second axis, the two axes may be substantially orthogonal. Typically, the body would be of doped silica material.
The passive means preferably comprises a feature which reduces the effective refractive index of the body material surrounding the core region such that there is a step change in refractive index between the core region and the surrounding body material in the first axis. The feature may comprise air channels, in particular a pair of air channels, spaced apart to either side of the core region thereby to define the first transverse axis. The air channels are typically of micron to sub-micron diameters. Hence, the passive means and the core region behave in the manner of a micro-structured optical fibre, specifically a solid core PCF or holey fibre. The passive means may comprise a plurality of features each promoting orientation of light in one first transverse axis of the body.
The active means may comprise a switchable feature which is switchable between an off state in which it has no effect on the refractive index of the core region and an on state in which it has an effect on the refractive index of the core region in the second axis. Preferably, the effect is an increase in the refractive index of the core region, further preferably such that the step change in the refractive index between the core region and the surrounding body material in the second axis is greater than the step change in the refractive index in the first axis. The switchable feature may comprise rods of material, preferably a pair of rods of material, spaced apart to either side of the core region thereby to define the second transverse axis. The pair of rods may be switchable between an off state in which they are unstressed and an on state in which they are stressed thereby to affect the refractive index of the core region. The stress may be applied by means of heat, electricity or mechanical force. The material for the rods will depend upon how the stress is to be applied but, for example, for heat induced stress, the rods may be of quartz based silica design. The rods may be several microns in diameter. Also preferably, the stress is applied by means of contacts applied to the outer surface of the body. Resistive heating of contacts is one way of inducing heat in the rods but other methods, such as electrical switching and travelling wave switching, are other examples. The active means may comprise a plurality of switchable each, in the on state, promoting orientation of light in one second axis.
In an alternative embodiment, the invention provides an orientation switchable optical fibre largely as the first aspect except that active means extends along the whole length of the body such that, according to the state of the active means, there are two possible orientations for light along the whole length of the fibre with switching between each facilitated.
According to a second aspect, the invention provides an orientation switchable optical fibre comprising an elongate body having two ends, which body is twisted intermediate the two ends, a central core region in which light carried by the fibre is confined, passive means extending substantially from the first end of the body to substantially the location of the twist, which passive means promotes orientation of the light in at least one first transverse axis of the body, active means extending substantially from the location of the twist to substantially the second end of the body, which active means has a plurality of features each switchable between an off state in which the feature is ineffective and an on state in which the feature is effective to promote orientation of light in a corresponding transverse axis of the body, wherein one of the second transverse axes is substantially aligned with a first of the first transverse axes and wherein the on state of the one of the plurality of features corresponding to the said one second axis light remains confined in the first axis along the length of the body and in the on state of one of the other of the plurality of features light transits on passing the location of the twist from the first axis to the second axis corresponding to the said one other of the plurality of features.
According to a third aspect, the invention provides an optical signal processor including an orientation switchable optical fibre according to a first or second aspect of the invention.
According to a fourth aspect, the invention provides an optical communications system including an orientation switchable optical fibre according to a first or second aspect of the invention or a processor according to a third aspect of the invention.
According to a fifth aspect, the invention provides a method of switching light orientation in an optical fibre comprising an elongate body having two ends, which body is twisted intermediate the two ends, a central core region in which light carried by the fibre is confined, passive means extending substantially from one end to substantially the other end of the body, which passive means promotes orientation of the light in at least one first transverse axis of the body, active means extending substantially from the location of the twist to substantially the other end of the body, which active means is switchable between an off state in which the active means is ineffective and an on state in which the active means is effective to promote orientation of light in at least one second transverse axis of the body, the method comprising switching the active means between the off state in which light remains confined in the at least one first axis along the length of the body and the on state in which light transits on passing the location of the twist from the at least one first axis to the at least one second axis.
In an alternative embodiment, the invention provides a method of switching light orientation in an optical fibre largely as described immediately above except that active means extends along the whole length of the body such that, according to the state of the active means, there are two possible orientations for light along the whole length of the fibre with switching between each facilitated.
According to a sixth aspect, the invention provides a method of switching light orientation in an optical fibre comprising an elongate body having two ends, which body is twisted intermediate the two ends, a central core region in which light carried by the fibre is confined, passive means extending substantially from the first end of the body to substantially the location of the twist, which passive means promotes orientation of the light in at least one first transverse axis of the body, active means extending substantially from the location of the twist to substantially the second end of the body, which active means has a plurality of features each switchable between an off state in which the feature is ineffective and an on state in which the feature is effective to promote orientation of light in a corresponding transverse axis of the body, wherein one of the second transverse axes is substantially aligned with a first of the first transverse axes, the method comprising switching one of the plurality of features corresponding to the said one second axis light to an on state such that light remains confined in the said first axis along the length of the body and alternately switching one of the other of the plurality of features in to an on state such that light transits on passing the location of the twist from the said first axis to a second axis corresponding to the said one other of the plurality of features.
According to a seventh aspect, the invention provides an optical signal processor including an orientation switchable optical fibre switched according to a fifth or sixth aspect of the invention.
According to an eighth aspect, the invention provides an optical communications system including an orientation switchable optical fibre switched according to a fifth or sixth aspect of the invention or a processor according to a seventh aspect of the invention.